The existing networks provide an access to internet mostly by ADSL and VDSL which employs a telephone line or a cable modem which employs a coaxial cable, or the like. Either the telephone line or the coaxial cable described above is comprised of copper wires so that a bandwidth thereof to be provided with subscribers has a maximum limit of approximately 10 Mb/s, although it may vary depending on a transmission distance. However, demand on a high-bandwidth service is rapidly increased as voice and text oriented services have evolved to video oriented services due to a rapid expansion of internet. As a method of meeting the demand on a high-bandwidth, an evolution of respective networks, which have been deployed by a company or companies running telecommunications businesses or cable TV businesses, is required for providing services where video, data and voice are integrated. In order to accommodate next-generation services including a high definition TV (HDTV)/an Internet Protocol TV (IP-TV), a video on demand (VOD), an education on demand (EOD), etc., which require a high bandwidth, a WDM-PON capable of providing the subscribers with a bandwidth of 100 Mb/s or more, while guaranteeing a high quality of service (QoS) is considered to be an ultimate alternative. Further, it is anticipated that a bandwidth for future optical networks will be increased gradually.
Generally, an arrayed wavelength grating (AWG) is widely used as a wavelength division multiplexing filter in a WDM-PON. However, since a wavelength to be assigned each subscriber and a transmission wavelength of an AWG may be changed when an ambient temperature is changed, a low-cost light source having a wavelength-independent operation, i.e., a color-free operation is necessarily required to be used as a light source independent of a wavelength assigned to each subscriber, in order to control and manage wavelength easily depending on a temperature change. As an example of a light source having a wavelength-independence characteristic described above, Hyun-Deok, Kim, et al. suggested a wavelength-locked Fabry-Perot laser diode (F-P LD) in an article entitled “A low-cost WDM source with an ASE injected Fabry-Perot semiconductor laser” published in August 2000 at IEEE Photon, Technol. Lett., vol. 12, no. 8, pp. 1067-1069. The wavelength-locked F-P LD suggested by Hyun-Deok, Kim, et al. is a method where a broadband light source (BLS) is injected into an F-P LD which oscillates in multiple modes so that the oscillated wavelength of the F-P LD is locked into the wavelength of the BLS. In this case, a high-power light emitting diode (LED), an erbium-doped fiber amplifier (EDFA) which emits amplified spontaneous emission (ASE), a super luminescent diode (SLD), etc., may be used as a BLS to be injected into a wavelength-locked F-P LD.
In the meanwhile, a light source to be used in an optical communications should have a low relative intensity noise (RIN) for maintaining a good quality of transmission. For example, an F-P LD which oscillates in multiple modes is not appropriate to be used as a light source for a WDM system or a WDM-PON, because transmission of an optical signal becomes difficult due to a high RIN thereof. More specifically, in case that one mode among multiple modes in an F-P LD is selected, a high mode partition noise occurs so that an F-P LD which oscillates in multiple modes is impossible to be used as a light source for WDM optical communications. As one method for reducing a RIN, a method for reducing a mode partition noise significantly by oscillating an F-P LD in a pseudo-single mode by injecting a BLS into the F-P LD (a wavelength-locked F-P LD) and a WDM-PON using the same have been suggested. However, even a WDM-PON using a wavelength-locked F-P LD bears a problem when accommodating lots of channels in one PON by narrowing a channel spacing or increasing a data rate as will be described below.
In a BLS which is injected into an F-P LD to embody a wavelength-locked F-P LD, a bandwidth of the BLS to be injected is determined by a bandwidth of an AWG being used. Thus, a BLS should have a high level of noise because it experiences a pre-filtering process when it is injected. Generally, as a data rate to be provided for each subscriber is higher, a light source to be used should have a better noise characteristic. However, there is a problem that a RIN of a BLS to be injected becomes poorer, as a bandwidth of a BLS or a channel spacing becomes narrower. This problem functions a factor barring a WDM-PON from accomplishing a larger capacity and a higher bandwidth thereof.
FIG. 6 illustrates a noise degradation characteristic measured before and after being filtered by an AWG of a wavelength-locked F-P LD in accordance with the present invention. Referring to FIG. 6, the RIN of a wavelength-locked F-P LD by injecting an ASE-based BLS which is filtered by using an AWG having a channel spacing of 50 GHz is measured at injection positions (detuning) of a BLS based on a lasing wavelength of an F-P LD as a standard, and is illustrated as rectangles. Here, the RIN of a total mode of the wavelength-locked F-P LD is illustrated as white rectangles, while the RIN of the light source which arrived at a receiving end after passing through AWGs for multiplexing and de-multiplexing is illustrated as black rectangles. As can be seen from FIG. 6, in case that a center wavelength of the BLS being injected conforms to a lasing wavelength of the F-P LD (0 nm detuning), a mode partition noise of the F-P LD is more effectively suppressed than a noise of the BLS being injected depending on an injection effect of the BLS, from a comparison of the RIN values between a spectrum sliced ASE and black rectangles. However, in case that a center wavelength of the BLS being injected lies between two lasing modes of the F-P LD (+/−0.3 nm detuning), a suppression effect of a mode partition noise of the F-P LD cannot be obtained and the noise of the wavelength-locked F-P LD is rather more increased than the RIN (−104 dB/Hz) of the injected BLS. Further, the RIN of depending on a temperature change of the wavelength-locked F-P LD may experience a worse noise degradation when passing through AWGs for multiplexing and de-multiplexing. As illustrated in FIG. 6, in case of using a wavelength-locked F-P LD by injection of an ASE-based BLS, the RIN (black rectangles) of a light source arriving at an receiving end, which is measured after passing through AWGs for multiplexing and de-multiplexing, does not always satisfy, with regard to 1.25 Gb/s data signal, the RIN value less than −107 dB/Hz which is a requirement for transmitting in a modulation format of an On Off Keying (OOK) modulation method, while satisfying a bit error rate (BER) of 10−12 (Q=7). This problem acts as an obstacle factor against a higher bandwidth and a larger capacity and appears commonly in both cases of using a wavelength-locked F-P LD as a light source for a WDM-PON and of using a reflective semiconductor optical amplifier (RSOA) having wavelength injection method.